A machine tool that performs turning includes a headstock having a chuck. The chuck can clamp an end of a workpiece such as a camshaft. A machine tool also includes a tailstock that is arranged to face the headstock. The tailstock can reciprocate and is pressed against a center hole in the free end of a workpiece. The tailstock is reciprocated via a lead screw mechanism. The lead screw mechanism is actuated by a servomotor. The tailstock is advanced by the servomotor via a screw shaft. When the tailstock center of the tailstock is pressed against the center hole of a workpiece, the workpiece is supported by the chuck and the tailstock center with a predetermined tailstock thrust.
Patent Document 1 discloses a method for setting a tailstock thrust of a tailstock center. According to this method, tailstock thrust is set by a tailstock thrust setting section. A torque command converting section converts the set tailstock thrust into drive torque of the servomotor, while taking into consideration the tailstock sliding surface resistance, the frictional torque of the receiving portion of the lead screw, and the efficiency of the screw shaft—these are set by a parameter setting section, and the gear ratio of the drive system. When the tailstock command section is operated, a tailstock controlling section is activated. Then, the torque of the servomotor is controlled through a servomotor controlling section and a power amplifying section, so that the tailstock is driven and controlled. The tailstock controlling section stores the drive torque, which has been obtained through conversion at the torque command converting section, in a torque limit value storing section, and sends a torque limit value to the servomotor controlling section. The servomotor controlling section performs torque limit control of the servomotor by setting a torque limit value (drive torque) as the upper limit. Accordingly, the tailstock thrust of the tailstock center is set.
In the method for setting the tailstock thrust in the above described conventional tailstock device, the torque value of the servomotor (current value) is limited before the tailstock center is pressed against a workpiece. Thereafter, the tailstock center is pressed against the workpiece. Accordingly, the tailstock cannot be operated when the torque limit value is set to a value less than or equal to the resistance such as the sliding surface resistance of the tailstock. Therefore, when a workpiece is made of a material of a low strength, such as a synthetic resin, the thrust of the tailstock center becomes excessive. This can deform the workpiece and lower machining accuracy of the workpiece by a tool.